1. Field of the Invention
The present invention relates to a sensor head which is used with a scale having a periodic optical pattern to constitute a reflective optical encoder.
2. Description of the Related Art
Currently, so-called encoders such as optical and magnetic encoders have been used to detect displacement amounts in a linear direction for machine tool stages, three-dimensional measurement instruments, and the like, and to detect rotational angles for servo motors and the like.
An optical encoder generally comprises a scale fixed to a displacement detection target such as a stage and a sensor head for detecting the displacement of the scale. The sensor head includes a light-emitting unit which applies light to the scale and a photodetection unit which detects a light beam modulated by the scale. The sensor head detects the movement of the scale in accordance with a change in the intensity of a received light beam.
An optical encoder has characteristics such as high precision, high resolution, noncontact, and high electromagnetic interference resistance, and hence is used in various fields. For applications demanding high precision and high resolution, in particular, the use of optical encoders goes mainstream.
A conventional representative optical encoder will be described with reference to FIG. 17. FIG. 17 shows a prior art of an optical encoder using an LED as a light source and a reflective scale. Such an optical encoder using an LED and reflective scale is disclosed in, for example, U.S. Pat. No. 5,995,229.
As shown in FIG. 17, this optical encoder comprises a reflective scale 502 and a sensor head 501 including an LED 520 and photodetector 533. The scale 502 has a periodic optical pattern on its surface. The optical pattern is formed by patterning a thin metal film made of chromium or the like on the surface of a transparent member such as a glass member. The sensor head 501 includes a transparent element substrate 530. A light source slit 534 is formed in the element substrate 530. The photodetector 533 is fixed on the transparent element substrate 530 and electrically connected to an interconnection 540 through an interconnection 512 formed on the surface of a resin-molded block 510. The LED 520 is fixed in an element embedding hole formed in the resin-molded block 510. An electrode 521 extending from the LED 520 is connected to the interconnection 540 through the interconnection 512 formed on the surface of the resin-molded block 510.
The scale 502 is fixed to a displacement detection target such as a stage (not shown), and moves relative to the sensor head 501 together with the displacement detection target. The sensor head 501 detects the movement amount or moving direction of the scale 502 on the basis of a change in the intensity of a light beam modulated by the scale 502. An output signal from the sensor head 501 is output as a waveform like that shown in FIG. 16. In this case, an A-phase signal and B-phase signal are a pair of waveforms output upon movement of the scale 502, and are generally quasi sinusoidal waves. The A-phase signal and B-phase signal are 90° out of phase with each other. The moving direction of the scale 502 can be detected from the phase relationship between the A-phase signal and the B-phase signal.
The operation of this optical encoder will be described next.
The LED 520 is connected to a power supply through the electrode 521, interconnection 512, and interconnection 540, and emits a light beam in accordance with the current supplied from the power supply. The light beam passes through the light source slit 534 and element substrate 530 and strikes the scale 502. The light beam reflected by the scale 502 is detected by the photodetector 533. At this time, when the pitch of the light source slit 534, the light beam wavelength of the LED 520, the pitch of the periodic pattern formed on the scale 502, the spacing between the sensor head 501 and the scale 502, and the like have a predetermined relationship, a bright-and-dark pattern similar to the periodic optical pattern formed on the scale 502 is projected on the photodetector 533. Therefore, the photodetector 533 detects the periodic bright-and-dark pattern and generates an A-phase signal and B-phase signal as a pair of signals having a phase difference of 90°.
A pitch p2 of the bright-and-dark pattern is calculated byp2=p1×(z1+z2)/z1  (1)where p1 is the pitch of the optical pattern formed on the scale 502, z1 is the spacing between the light source slit 534 and the scale 502, and z2 is the spacing between the scale 502 and the light-receiving surface of the photodetector 533.
If z1 and z2 are equal to each other in equation (1),p2=2×p1  (2)
That is, if the distance between the scale 502 and the light source slit 534 is made equal to the distance between the scale 502 and the photodetector 533, the pitch p2 of the bright-and-dark pattern does not change even with a change in the spacing between the scale 502 and the sensor head 501. This makes it possible to perform detection more stably. In other words, in order to perform stable detection, the level of the light source slit 534 relative to the scale 502 is preferably equal to that of the photodetector 533. Since the bright-and-dark pattern moves on the photodetector 533 upon movement of the scale 502, the movement of the scale 502 can be detected by detecting the movement of the bright-and-dark pattern.
In general, when the sensor head 501 is to be mounted, since the positional relationship between the scale 502 and the sensor head 501 must be precisely adjusted, the bottom surface (the upper surface in FIG. 17) of the sensor head 501 is often used as an abutting surface. According to the sensor head described above, however, since the electrode 521 of the LED 520 is formed on the bottom surface of the resin-molded block 510, the electrode 521 protrudes from the bottom surface of the resin-molded block 510. This tends to cause a shift in level or a tilt when the sensor head is mounted.